Amphibole Group
=Amphibole Group= The Amphibole group is a categorization of rock-forming inosilicate minerals. They are inosilicates because Amphiboles are chain silicates with tetrahedral organization.They are composed of double-chains of SiO4 tetrahedra (hence the name "double-chain silicate"). Amphiboles are found in many igneous and metamorphic rocks. They can only form in environments where water is present, as opposed to the single-chained pyroxenes. Because of this difference, scientists can examine the percent difference between pyroxenes and amphiboles in a rock to determine the amount of water present during formation. General Formula: W0-1X2Y5Z8O22(OH)2 where W cations (blank, Na, K) occupy the A sites, X cations (Ca, Mg, Fe2+) occupy the M4 sites, Y cations (Mg, Fe2+, Al3+, Fe3+) occupy the M1, M2, and M3 sites, and Z cations (Si4+, Al3+) occupy the tetrahedral site.1 There are five groups of amphiboles. These are: - Cummingstone Subgroup: Fe-Mg-Li Clinoamphiboles - Anthophyllite Subgroup: Fe-Mg-Li Orthoamphiboles, forms from metamorphism of magnesium-rich rocks - Tremolite Subgroup: Ca Amphiboles, forms from metamorphism of dolomite and quartz-rich sediments - Richterite Subgroup: Ca-Na Clinoamphiboles, forms from thermally metamorphosed limestones in contact metamorphic zones - Glaucophane Subgroup: Alkali Amphiboles, formed in Na-rich environments, associated with subduction zone chemistry Distinguishing Features: *120° cleavage *Moderate to high relief *Biaxial *Medium interference colors *Fibrous habit Photomicrograph of glaucophane in an eclogite under plain polarized light, showing distinct color and 120° cleavage. Tremolite (as distinct from Actinolite): *No pleochroism *Colorless in PP (whereas Actinolite can be light brown to light green) Photomicrograph of Tremolite grain in a chlorite schist under plain polarized light. Photomicrograph of same grain in crossed polarized light. 1 Nesse, W. D. (2000). Introduction to mineralogy. Oxford, NY: Oxford University Press. Cummingtonite *☐{Mg2}{Mg5}(Si8O22)(OH)2 *Physical Properties #Luster: Vitreous #Color: Translucent #Hardness: 5-6 #Cleavage: Perfect on {110} *Monoclinic *Unit Cell Dimensions: a = 9.53Å, b = 18.23Å, c = 5.32Å, β = 101.97° *Unit Cell Volume: 904.16 Å³ *Biaxial (+) *Refractive Indices: n''α = 1.639 - 1.671 ''nβ = 1.647 - 1.689 n''γ = 1.664 - 1.708 *2V: 65° to 90° *Maximum Birefringence: 0.025-0.037 *Moderate relief Glaucophane *◻Na2Mg3Al2Si8O22(OH)2 *Physical Properties: #Luster: Vitreous #Color: Grey-lavender #Streak: grey/bluish grey #Hardness: 5-6 *Monoclinic *Unit Cell Dimensions: a = 9.59Å, b = 17.79Å, c = 5.3Å, β = 103.66° *Unit Cell Volume: 878.64 Å³ *Biaxial (-) *Refractive Indices: ''nα = 1.606 - 1.637 n''β = 1.615 - 1.650 ''nγ = 1.627 - 1.655 *2V: 10° to 80° *Maximum Birefringence: 0.021 *Moderate relief Richterite *{Na}{CaNa}{Mg5}(Si8O22)(OH)2 *Physical Properties: #Luster: Vitreous #Translucent #Color: Brown, red, grey-brown #Streak: White #Hardness: 5-6 *Monoclinic *Unit Cell Dimensions: a = 9.9Å, b = 17.97Å, c = 5.26Å, β = 104.25° *Unit Cell Volume: 906.98 Å³ *Biaxial (-) *Refractive Indices: n''α = 1.615 ''nβ = 1.629 n''γ = 1.636 *2V: 68° *Maximum Birefringence: 0.021 *Moderate relief Anthophyllite ☐{Mg2}{Mg5}(Si8O22)(OH)2 *Physical Properties #Luster: Vitreous, pearly #Color: white, green, grey, brown #Transparent, translucent #Streak: White/grey #Hardness: 5.5-6 #Texture: Long sharp fexible fibers #Cleavage: Perfect on {210} #Fracture: Conchoidal #Measured density: 2.85-3.57 g/cm3 *Orthorhombic *Cell Unit Dimensions: a = 18.5Å, b = 17.9Å, c = 5.28Å *Unit Cell Volume: 1,748.47 Å³ *Biaxial (+) *Refractive Indices: ''nα = 1.598 - 1.674 n''β = 1.605 - 1.685 ''nγ = 1.615 - 1.697 *2V: 57° to 90° *Maximum Birefringence: 0.017-0.023 *Moderate relief Tremolite #Luster: Vitreous, silky #Color: White, brown, colorless, grey, light green, light yellow, pink #Streak: White #Hardness: 5-6 #Cleavage: Perfect on {110} #Transparent, translucent #Measured density: 2.99-3.03 g/cm3 *Monoclinic *Unit cell dimensions: a = 9.84Å, b = 18.02Å, c = 5.27Å, β = 104.95° *Unit cell volume: V=902.83 Å³ *Simple and multiple twinning common parallel to {100} *Biaxial (-) *Refractive indices: n''α = 1.599 - 1.612 ''nβ = 1.613 - 1.626 nγ = 1.625 - 1.637 *2V: 88° to 80° *Maximum birefringence: 0.026 *Moderate relief *Occurs as a result of metamorphosed ultramafic rocks and magnesium carbonate rocks. Also forms during contact metamorphism of Ca-Mg siliceous material. Clee04 00:44, January 28, 2011 (UTC) Reference: http://www.mindat.org/gallery.php?min=4011 Category:Tremolite